Federal regulations limit the level or amount of noise that may be emitted from a jet engine. Thus, various methods and designs have been proposed for attenuating noise in flow ducts of aircraft engines. One such design uses an acoustic lining system. This system relies on the open architectures of acoustic suppression materials to act as Helmholtz resonators. The acoustic liner includes a sound permeable facing sheet and a sound impermeable backing sheet sandwiched around a honeycomb core. The facing sheet is generally formed from sintered metal mesh and is attached to a perforated liner. The perforations are sized and spaced such that the liner is tuned to the noise frequency generated at different locations in the engine. The honeycomb core dissipates acoustical energy after passing through the metal mesh and tuned liner. The sound impermeable backing resists acoustical energy radiation, thereby preventing sound transmission.
The laminar sound attenuator described above suffers from several drawbacks. The sound frequency in the jet engine duct varies throughout the duct. Thus, the liner must be precisely tuned to the varying frequencies to take advantage of the Helmholtz resonator effect. As a result, the attenuation level drops severely if frequencies change or if the liner is not precisely tuned. Many factors, including engine model and fan speed, affect the discrete primary frequencies which cause the noise in jet engines. Consequently, it is difficult to design a precisely tuned liner without incurring unreasonable expense and risk of an inoperative or ineffective liner. Other drawbacks include high speed grazing flow effects and degradation due to high temperatures.
Recent designs have attempted to broaden the frequency range over which these noise attenuators effectively attenuate sound waves. One such design provides plural layers of permeable sheets and honeycomb cores. Although this design achieves a broader sound attenuating characteristic, the liners are bulky and heavy and difficult to manufacture for effective use in jet engines. Another design involves modifying the shape and design of the honeycomb structure. The resulting complex honeycombs are difficult and expensive to manufacture.
U.S. Pat. No. 3,734,234 to Wirt discloses a sound absorbing structure having a honeycomb like cellular layer, an impermeable backing sheet, a permeable facing sheet, and oblique porous partition members having a specified flow resistance in each cell. FIGS. 10 and 16 show oblique partition members disposed between parallel walls. These members act in a similar manner as if a bulk material were disposed between the parallel walls. The parallel walls are not spaced apart from one another so as to function as noise attenuators, but instead merely act as wave guides, with the oblique members functioning as noise attenuators.
Another design for attenuating sound over a broader frequency band is disclosed in U.S. Pat. No. 5,014,815 to Arcas et al. The acoustic liner disclosed therein comprises a sound permeable inside plate forming a first closed annulus and a second impermeable outside plate forming a second closed annulus. The inside and outside plates are spaced apart and thus form an annulus chamber therebetween in which a core member is secured. The core member has a sinusoidal shape and extends annularly around the inside plate. The resulting variable depth sound absorption chambers attenuate sound waves over a substantially broader frequency range.
U.S. Pat. Nos. 4,848,514, 4,926,963 and 4,947,958 to Snyder disclose sound attenuating laminates consisting of several layers, each of which assists in suppressing noise. Hollow rivets are used for installation purposes and conduct acoustical energy to the intermediary layers of the noise attenuating laminate. The above-described patents all suffer from the same drawbacks that result from the layer type structure such as difficulty in precisely tuning the liner and expense and intricacy of manufacture.
Another type of sound attenuator is disclosed in a design for a gun baffler or silencer. These devices have deflecting members which dissipate sound by slowing or diverting expanding gasses. In a high temperature, high speed flow duct, such as an aircraft engine, such an attenuator would severely hamper the efficiency and flow through the duct, thereby rendering the device inoperable.
It is also known that bulk type materials, such as fiberglass or foams, installed in resonator cavities, enhance broadband noise attenuation. Bulk material, however, is structurally unacceptable in most aircraft applications due to its size and its inability to withstand high temperatures.